Gas turbine engines, such as those utilized in aircrafts, operate at high temperatures to maximize efficiency and thrust. Such temperatures can place the engine, its various modulated systems, and parts thereof, under tremendous thermal stress, which can lead to fatigue, wear, and in some instances, failure.
Internal and external fluid velocities of gas turbine engines can impact both individual parts as well as overall system efficiency. However, measuring fluid flow in components having complex internal passages is often complicated by the fact that such devices typically have multiple regions of flow separation and strong secondary flows. To fully understand these flows, three-dimensional (3D) fluid velocity data must be collected for a large number of points. In some engine tests, it is not uncommon for several hundred flow measurement instruments to be used to obtain enough data points for the full fluid flow of the engine to be properly studied and characterized. However, this large number of instruments can lead to technical problems such as fluid flow blockage due to the intrusiveness of the instruments.
Another means by which to obtain this data is computational fluid dynamics, or CFD. Unfortunately, the computer resource requirements and the fidelity of boundary conditions needed to quantify the 3D fluid flow through a complex passage can often be limiting factors. Other flow measuring systems currently used in conjunction with gas turbine engines generally include two-dimensional techniques such as Pitot tube, hot wire, Laser Doppler Anemometry (LDA) and Particle Imaging Velocimetry (PIV), each of which may have associated issues. For example, the Pitot tube and hot wire devices may often be intrusive and the measurement provided is point-by-point rather than encompassing the entire fluid flow field. In contrast, while both LDA and PIV may be non-intrusive techniques, both require particle seeding, which uses light to trace the path of particles placed in the fluid flow and works on the assumption that the particles will follow the fluid flow. Thus, there is a need for improved fluid measurement techniques.
Recently, four dimensional magnetic resonance velocimetry, or MRV, such as that used in medical diagnostic applications and full body scans, has been investigated for its effectiveness in measuring internal fluid flow. See Elkins, C. J. et al., 4D Magnetic resonance velocimetry for mean velocity measurements in complex turbulent flows, Experiments in Fluids, 34 (2003) pp. 494-503. MRV can be a desirable alternative to the previously discussed techniques as it is non-invasive and has been used in the past for both low and high Reynolds number studies. See Elkins citing Fukushima, E., Nuclear magnetic resonance as a tool to study flow, Annual Review of Fluid Mechanics, 31 (1999) pp. 95-123). MRV has been coupled with the rapid prototyping processes of stereolithography and fused deposition machining to study turbulent flows in complex internal geometries, specifically within a serpentine cooling passage of a stationary turbine blade. See Id.
MRV works by utilizing a four-dimensional pulse sequence. See Elkins citing Markl, M. et al., Time resolved three dimensional phase contrast MRI (4D-flow), Journal of Magnetic Resonance Imaging (2003). In general, MRV involves using both a 3D cine sequence and 3-directional velocity encoding to simultaneously generate a time-resolved series of 3D magnitude images and three-component velocity information. Within each gating cycle, one spatial phase encoding step along the y-direction is used for all acquired time frames. All velocity measurements necessary for the three-directional flow information and a selectable number of spatial encodings along the z-direction are interleaved. The net result is a flexible trade-off between temporal resolution of the images and total acquisition time. See Id.
MRV does, however, still produce data errors that need to be addressed. For instance, to reduce velocity errors unrelated to fluid flow, two scans—one with the flow on and one with the flow off—can be used. Eddy currents and other sources of off-resonance effects, such as gradient field-related system imperfections, can lead to errors in velocity measurements. To account for these errors, the entire measurement process can be repeated without flow but with otherwise identical parameters. Subtraction from the data set containing the velocity information can then be used to eliminate cumulative velocity errors. See Elkins citing Markl, M. et al., Analysis and correction of the effect of spatial field distortions on velocity measurements with phase contrast MRI, Proc. Of the 10th Scientific Meeting of the International Society for Magnetic Resonance in Medicine, Honolulu, USA, May 18-24, 2002.
Additionally, nonlinearities in the gradient magnetic fields used to encode the velocity can result in errors in the velocity maps. Such imperfections can introduce errors in velocity measurements by altering the gradient moment used to encode flow or motion. Any change from the ideal local gradient can be directly reflected by the same percentage change in the gradient moment and thus, the velocity encoding. However, since the spatial distortions of the gradient field are known, the measured velocities can be retrospectively corrected. See Id.
Thus, while current MRV practices allow internal flow velocities of complex stationary passages to be studied, there remains a need for similarly non-intrusive methods by which to measure and optimize the fluid flow path of the entire gas turbine engine, as well as modulated engine systems and parts thereof, including both stationary and rotatable components.